1. Technical Field
The subject matter described here generally relates to fluid reaction surfaces with specific blade structures, and, more particularly, to wind turbines having vortex breakers near the tip of the blades.
2. Related Art
A wind turbine is a machine for converting the kinetic energy in wind into mechanical energy. If the mechanical energy is used directly by the machinery, such as to pump water or to grind wheat, then the wind turbine may be referred to as a windmill. Similarly, if the mechanical energy is converted to electricity, then the machine may also be referred to as a wind generator or wind power plant.
Wind turbines use an airfoil in the form of a blade to generate lift and capture momentum from moving air that is them imparted to a rotor. The blade is typically secured to a rotor at its root end, and extends radially to free, tip end. The front, or leading edge, of the blade connects the forward-most points of the blade that first contact the air. The rear, or trailing edge, of the blade is where airflow that has been separated by the leading edge rejoins after passing over the suction and pressure surfaces of the blade. A chord line connects the leading and trailing edges of the blade in the direction of the typical airflow across the blade. The length of a chord line is simply referred to simply as the chord.
The outboard ends of the blade are called tips and the distance from the tip to the root, at the opposite end of the blade, is called the span. Since many blades change their chord over the span from root to tip, the chord length is referred to as the root chord, near the root, and the tip chord, near the tip of the blade. The resulting shape of the blade, when viewed perpendicular to the direction of flow, is called the planform. Since the thickness of a blade will typically vary across the planform, the term thickness is typically used to describe the maximum distance between the low pressure suction surface and the high pressure surface on the opposite side of the blade. As with other airfoils, wind turbine blades are sometimes provided with flat, usually thin, plates attached at one edge, and referred to as flaps.
Wind turbines are typically categorized according to the vertical or horizontal axis about which the turbine rotates. Horizontal configurations are most common in modern wind turbine machines and one so-called horizontal-axis wind generator is schematically illustrated in FIG. 1, copied from U.S. Pat. No. 7,144,216. This particular configuration for a wind turbine 1 includes a tower 2 supporting a drive train 4 with a rotor 6 that is covered by a protective enclosure referred to as a nacelle. Blades 8 are arranged at one end of the rotor 6 outside the nacelle for driving a gearbox 10 and electrical generator 12 at the other end of the drive train 4 inside the nacelle.
The “upwind” configuration shown in FIG. 1, where the rotor 6 faces into the wind, helps to reduce noise by eliminating the wind shadow of the tower 2 that can other result in an impulsive thumping sound. Other aspects of wind turbine noise have also been addressed by providing quieter gearboxes, soundproofed nacelles, and streamlined nacelles and towers. More efficient blade designs that convert less of the wind's energy into aerodynamic noise have also been sought.
For example, U.S. Patent Publication No. 20060216153 discloses a rotor blade for a wind power plant with a tip that is curved or angled at it end region in the direction of the pressure side of the blade. In order further to reduce the levels of sound emission, the blade is curved or angled in its edge region in the direction of the trailing edge of the rotor blade in the plane of the rotor blade. An English-language abstract of World Intellectual Property Organization Publication No. 2006059472 also discloses a propeller wherein the tip parts of the propeller blades of a horizontal-shaft windmill are tilted in the front direction of the propeller blades. However, these and other conventional approaches to blade tip configuration do not adequately address the problems of aerodynamic efficiency and wing tip noise because they can actually increase blade drag which correspondingly reduces turbine power.